The wavefront of an optical wave is the locus of points that have the same phases, i.e. points that have the same optical path length from a light source from which the optical wave originates. Wavefront sensors are employed to measure and analyse the wavefront of an optical wave. Measuring and analysing the wavefront of an optical wave is applied in a number of different technologies, including measurement of the flatness or warpage of wafers or chucks in semiconductor technology, or measurement of the flatness of work pieces in precision engineering, such as for LCD panels, hard disks etc. Other areas of application include measurement of the aberration of optics and beam quality of lasers in optics manufacture, measurement and analysis of aberrations of eyes in ophthalmic diagnosis, and measurement and analysis of the turbulence of the atmosphere to improve the quality of observations in astronomy and adaptive optics technology fields.
The technologies in wavefront sensors include Hartmann wavefront sensors (HWS), and Shack-Hartmann wavefront sensors (SHWS). In the original HWS technology, a micro-holes array 100 is positioned in front of an optical lens 102, as shown in FIG. 1. The micro-holes array 100 is used to measure the slopes of a wavefront 104 in sub-apertures. The wavefront is reconstructed based on the matrix of slopes measured at detection plane 106. The reconstruction algorithms are generally categorized into zonal and modal estimations.
In SHWS, which constituted a break-through of wavefront sensor technology, a lenslet array 200 is employed, as shown in FIG. 2. The lenslet array 200 replaces the micro-holes array 100 (FIG. 1) and lens 102 (FIG. 1), by providing an array of physical sub-apertures, i.e. lenslets in the one optical component. The wavefront reconstruction algorithms employed are the same as for HWS. Most current commercial wavefront sensors are SHWS.
In order to improve the dynamic range of SHWS, it has been suggested to employ a spatial light modulator (SLM) as a physical shutter array aligned with the lenslet array 200. The SLM is placed in front of the physical lenslet array 200 and used to switch on and off the sub-apertures of the lenslet array 200, which remains the focusing element of the SHWS.
It has also been suggested to employ an SLM in HWS, to develop a scanning HWS. The SLM is used to generate a micro-holes array that is moved in a lateral direction relative to the lens 102 (FIG. 1) to improve the sensitivity, dynamic range and accuracy of the HWS. The focusing element remains the physical lens 102 (FIG. 1) in scanning HWS.
A physical scanning technique has also been proposed for SHWS, in which the physical lenslet array 200 and detection plane (FIG. 2) are shifted in both x and y directions. This physical scanning technique can be used to improve the measurement range in the lateral direction, where the measurement results of every sub-area are stitched to form a larger area.
The main drawbacks of conventional HWS include that the contrast of the light beam projected on the photoelectric detectors is poor, resulting in poor accuracy of centroid finding and wavefront reconstruction. The vignetting effect induces a larger spot size and consequently a relatively small measurement range. On the other hand, the drawbacks of conventional SHWS include the use of a physical lenslet array. The features and parameters of conventional SHWS are thus fixed and restricted by the physical lenslet array. Since the lateral resolution is depend on the size of lenslet, generally, another drawback of conventional SHWS is the poor lateral resolution.
Even with the proposed modifications to both HWS and SHWS as described above, there are significant drawbacks. More particular, the proposed modifications to HWS still exhibit the drawbacks of HWS mentioned above. In relation to modifications of SHWS, the main drawback remains that those modifications have been unable to achieve an improved lateral resolution without introduction of potentially serious measurement errors as a result of physical movement of entire components of the SHWS.
A need therefore exists to provide a wavefront sensor that addresses at least one of the above mentioned drawbacks.